The present invention generally relates to digital circuitry, and more particularly to an apparatus for controlling access to a digital timer circuit.
In the field of digital computers and devices, a day and date timer circuit is often disposed for providing a time reference, either for the operator or for execution of programs. From time to time it may be necessary to reset the timer or to provide a read-out of the contents thereof, on a bus for example. The timer circuit disclosed herein is fabricated in a conventional manner by a concatenation of flip-flop circuits, whereby the output of one flip-flop is coupled to the input of the next flip-flop, et seq. Also, the output of each of the flip-flops forms a pars of the output of the timer circuit. Moreover, a clock signal is provided in parallel to each of the flip-flop circuits.
To be useful in a digital computer, for example, the timer must be large enough for long counts. As a result, the timer is so large that it is impractical to synchronize carry signals to the next stage by conventional means. Hence, the timer would not function properly. For example, if the period of time to perform a carry operation is longer than the clock period, then it is obvious that the timer cannot count every clock.
It is possible for the timer to be accessed (i.e., read or loaded) on a transition of the clock signal driving the timer. Moreover, during normal operation of the timer, a brief period of instability immediately follows each incrementation of the timer. Thus, a problem may occur if the timer is read during this period of instability.